as for first timers??? plenty of forplay.. fingers in the ass get you reLaxed... and the best position for first time is

girl lies on her stomach guy gets on from behind... sort of a backward missionary possition... a pillow under the pelvis to help angle is also a help... in no time at all u will be doin it every which way

just tell him to keep the thrusts slow and STRAIGHT until you both feel comfie to step up the pace, depth etc

if he can play with your clitoris at the same time it helps relax and keep u stimulated... well it works for me